1. Field of the Invention
The present invention relates to the production of elemental silicon crystals useful in the fabrication of a variety of photovoltaic devices, and, more especially, relates to the preparation of polycrystalline silicon in a thin layer by reduction of a silicon containing compound.
2. Description of the Prior Art
The preparation of high purity silicon by reduction of silicon tetrachloride with zinc in the vapor phase is known [Lyon, Olson & Lewis, Trans. Electrochem. Soc., 96, 359 (1949)]. Such reaction takes place at 950.degree. C. and the resultant crystals are needle shaped. The process has the disadvantages of requiring high reaction temperatures, is difficult to operate because the presence of zinc in the vapor phase mandates use of a vaporizer, and also by reason of the fact that the resultant product finds no direct use, since the silicon obtained is in the acicular form and must be remelted at temperatures in excess of 1400.degree. C. to be usable.
In Yoshizawa et al, Chemical Abstracts, 57, 3076i-3077b (1962), silicon tetrachloride mixed with a carrier gas is reduced with molten aluminum, the silicon from the reaction being dissolved in the molten aluminum, and then crystallized to powders and plates on cooling. At high temperatures, vapor-phase reduction is said to have occurred to a small extent, with needle and dendritic crystals being formed. When the yield of Si is independent of the flow rate of the SiCl.sub.4, after the formation of a thin layer of Si on the surface of the molten aluminum, the reaction is limited by the diffusion of Si in molten Al.